1. Field of the Invention
This invention relates to the Air Traffic Control Radar Beacon System, and more particularly to a method for characterizing the system antenna patterns during actual use.
2. Description of the Related Art
In the United States, the Federal Aviation Administration uses a radio navigation system known as the Air Traffic Control Radar Beacon System (ATCRBS) to determine the range and azimuth of transponder-equipped aircraft. The ATCRBS consists of a ground-based interrogator-receiver that interrogates the aircraft transponder and is unique in its use of two separate radio emission patterns, known as antenna patterns, to improve the accuracy of the measured aircraft locations. The radar beacon system depends on a precise relationship between these two antenna patterns for satisfactory performance. The emissions for each pattern consists of a series of pulses which are transmitted at separate times but on the same radio frequency, thus precluding the use of conventional antenna pattern measurement techniques to characterize the two patterns.
One of the signals emitted by the ATCRBS system is roughly the same strength in all directions from the antenna and is therefore known as the omni-directional signal. The second signal emitted is a directional signal which is much stronger in one direction than any other. The direction of maximum strength is called the main lobe of the antenna, and the antenna is rotated either physically or electrically so that all azimuths are swept by this signal during a fixed period of time. Both the omni-directional and directional signals are emitted at the same frequency, 1030 MHz, but are pulsed so that they are separated in time. A standard sidelobe suppression system (SLS) emits an initial pulse from the directional antenna, followed by a pulse from the omni-directional antenna, and then one or more additional pulses from the directional antenna. A variation called the improved sidelobe suppression system (ISLS) differs in that it emits the first pulse from both antennas. This helps remove azimuth ambiguities caused by signal reflections.
Conventional receiving equipment is unable to separate the pulses emitted by the directional antenna from those emitted by the omni-directional antenna. In order to measure the antenna patterns of the two antennas using these systems, each of the signals must be measured separately with the other turned off, degrading air traffic control information. It is possible to use a pulse separating system connected to the receiver to measure both signals simultaneously, but this is bulky and expensive and is therefore seldom used. Disrupting the service of the ATCRBS system or using expensive specialized equipment are currently the only methods available for making these measurements.